The present disclosure relates to a skew detection method and an optical disc device applied to, for example, an optical disc.
In the related art, an optical disc capable of recording information or reproducing the recorded information by using laser light is increasingly used. In order to increase a surface recording density, the beam spot diameter is narrowed by increasing the numerical aperture (NA) of an objective lens. However, the increase in numerical apertures leads to the increase in aberration. In particular, there is an increase in aberration which occurs at the time when the angle between an optical axis of an objective lens and a surface of an optical disc deviates from 90 degrees due to warpage or the like of the optical disc. Such aberration produces distortion of light spot on a surface of the disk and thus blurring is problematic. As a result, there is a problem in that crosstalk from adjacent tracks increases and thus the quality of reproduction signals deteriorates.
In this case, the distortion is in proportion to the distance over which light passes through the optical disc, i.e., the thickness of a cover layer, thus the thickness of the cover layer is reduced to 0.1 millimeters (mm) in the Blu-ray Disc (registered trademark). However, skew of the optical disc (also referred to as tilt or the like) occurs due to warpage or the like of the optical disc, and thus it is necessary to detect and correct skew of the optical disc. In an embodiment of the present disclosure, it is intended to correct skew in the radial direction of the disk (the direction transverse to a track).
As a method of detecting skew of an optical disc, there is a method that employs a skew sensor. However, the skew sensor makes the configuration more complex, and it is disadvantageous in terms of costs. A technique for using a reproduction signal of an optical disc without the skew sensor has been proposed (refer to Japanese Unexamined Patent Application Publication No. 2001-266385).
Japanese Unexamined Patent Application Publication No. 2001-266385 discloses a technique in which a main beam is irradiated onto the center of reproduction information tracks, two sub beams are irradiated onto a position deviated outward from the center of the reproduction information tracks by a predetermined distance, and reflected light of the two sub beams are received by a four-divided light receiving element. The output of the four-divided light receivingelement is calculated to obtain the amount of skew. A coma aberration is generated due to a tilt, and the reproduction signal values are influenced by both distortion of the reproduction signal itself and crosstalk from adjacent tracks. The distortion of the reproduction signal is due to the fact that a beam spot becomes an symmetrical shape with respect to the tilt direction and crosstalk from adjacent tracks.